Process and apparatus for obtaining bulk monocrystalline gallium-containing nitride

ABSTRACT

The present invention refers to an ammonobasic method for preparing a gallium-containing nitride crystal, in which gallium-containing feedstock is crystallized on at least one crystallization seed in the presence of an alkali metal-containing component in a supercritical nitrogen-containing solvent. The method can provide monocrystalline gallium-containing nitride crystals having a very high quality.

The present invention refers to processes for obtaining agallium-containing nitride crystal by an ammonobasic method as well asthe gallium-containing nitride crystal itself. Furthermore, an apparatusfor conducting the various methods is disclosed.

Optoelectronic devices based on nitrides are usually manufactured onsapphire or silicon carbide substrates that differ from the depositednitride layers (so-called heteroepitaxy). In the most often usedMetallo-Organic Chemical Vapor Deposition (MOCVD) method, the depositionof GaN is performed from ammonia and organometallic compounds in the gasphase and the growth rates achieved make it impossible to provide a bulklayer. The application of a buffer layer reduces the dislocationdensity, but not more than to approx. 10⁸/cm². Another method has alsobeen proposed for obtaining bulk monocrystalline gallium nitride. Thismethod consists of an epitaxial deposition employing halides in a vaporphase and is called Halide Vapor Phase Epitaxy (HVPE) [“Opticalpatterning of GaN films” M. K. Kelly, O. Ambacher, Appl. Phys. Lett. 69(12) (1996) and “Fabrication of thin-film InGaN light-emitting diodemembranes” W. S. Wrong, T. Sands, Appl. Phys. Lett. 75 (10) (1999)].This method allows for the preparation of GaN substrates having a 2-inch(5 cm) diameter.

However, their quality is not sufficient for laser diodes, because thedislocation density continues to be approx. 10⁷ to approx. 10⁹/cm².Recently, the method of Epitaxial Lateral OverGrowth (ELOG) has beenused for reducing the dislocation density. In this method the GaN layeris first grown on a sapphire substrate and then a layer with SiO₂ isdeposited on it in the form of strips or a lattice. On the thus preparedsubstrate, in turn, the lateral growth of GaN may be carried out leadingto a dislocation density of approx. 10⁷/cm².

The growth of bulk crystals of gallium nitride and other metals of groupXIII (IUPAC, 1989) is extremely difficult. Standard methods ofcrystallization from melt and sublimation methods are not applicablebecause of the decomposition of the nitrides into metals and N₂. In theHigh Nitrogen Pressure (HNP) method [“Prospects for high-pressurecrystal growth of III–V nitrides” S. Porowski et al., Inst. Phys. Conf.Series, 137, 369 (1998)] this decomposition is inhibited by the use ofnitrogen under the high pressure. The growth of crystals is carried outin molten gallium, i.e. in the liquid phase, resulting in the productionof GaN platelets about 10 mm in size. Sufficient solubility of nitrogenin gallium requires temperatures of about 1500° C. and nitrogenpressures in the order of 15 kbar.

The use of supercritical ammonia has been proposed to lower thetemperature and decrease the pressure during the growth process ofnitrides. Peters has described the ammonothermal synthesis of aluminiumnitride [J. Cryst. Growth 104, 411–418 (1990)]. R. Dwilinski et al. haveshown, in particular, that it is possible to obtain a fine-crystallinegallium nitride by a synthesis from gallium and ammonia, provided thatthe latter contains alkali metal amides (KNH₂ or LiNH₂). The processeswere conducted at temperatures of up to 550° C. and under a pressure of5 kbar, yielding crystals about 5 μm in size [“AMMONO method of BN, AlN,and GaN synthesis and crystal growth”, Proc. EGW-3, Warsaw, Jun. 22–24,1998, MRS Internet Journal of Nitride Semiconductor Research,http://nsr.mij.mrs.org/3/25]. Another supercritical ammonia method,where a fine-crystalline GaN is used as a feedstock together with amineralizer consisting of an amide (KNH₂) and a halide (KI) alsoprovided for recrystallization of gallium nitride [“Crystal growth ofgallium nitride in supercritical ammonia” J. W. Kolis et al., J. Cryst.Growth 222, 431–434 (2001)]. The recrystallization process conducted at400° C. and 3.4 kbar resulted in GaN crystals about 0.5 mm in size. Asimilar method has also been described in Mat. Res. Soc. Symp. Proc.Vol. 495, 367–372 (1998) by J. W. Kolis et al. However, using thesesupercritical ammonia processes, no production of bulk monocrystallinewas achieved because no chemical transport processes were observed inthe supercritical solution, in particular no growth on seeds wasconducted.

Therefore, there was a need for an improved method of preparing agallium-containing nitride crystal.

The lifetime of optical semiconductor devices depends primarily on thecrystalline quality of the optically active layers, and especially onthe surface dislocation density. In case of GaN based laser diodes, itis beneficial to lower the dislocation density in the GaN substratelayer to less than 10⁶/cm², and this has been extremely difficult toachieve using the methods known so far. Therefore, there was a need forgallium-containing nitride crystals having a quality suitable for use assubstrates for optoelectronics.

The subject matter of the present invention is recited in the appendedclaims. In particular, in one embodiment the present invention refers toa process for obtaining a gallium-containing nitride crystal, comprisingthe steps of:

-   (i) providing a gallium-containing feedstock, an alkali    metal-containing component; at least one crystallization seed and a    nitrogen-containing solvent in at least one container;-   (ii) bringing the nitrogen-containing solvent into a supercritical    state;-   (iii) at least partially dissolving the gallium-containing feedstock    at a first temperature and at a first pressure; and-   (iv) crystallizing gallium-containing nitride on the crystallization    seed at a second temperature and at a second pressure while the    nitrogen-containing solvent is in the supercritical state;    wherein at least one of the following criteria is fulfilled:-   (a) the second temperature is higher than the first temperature; and-   (b) the second pressure is lower than the first pressure.

In a second embodiment a process for preparing a gallium-containingnitride crystal is described which comprises the steps of:

-   (i) providing a gallium-containing feedstock comprising at least two    different components, an alkali metal-containing component, at least    one crystallization seed and a nitrogen-containing solvent in a    container having a dissolution zone and a crystallization zone,    whereby the gallium-containing feedstock is provided in the    dissolution zone and the at least one crystallization seed is    provided in the crystallization zone;-   (ii) subsequently bringing the nitrogen-containing solvent into a    supercritical state;-   (iii) subsequently partially dissolving the gallium-containing    feedstock at a dissolution temperature and at a dissolution pressure    in the dissolution zone, whereby a first component of the    gallium-containing feedstock is substantially completely dissolved    and a second component of the gallium-containing feedstock as well    as the crystallization seed remain substantially undissolved so that    an undersaturated or saturated solution with respect to    gallium-containing nitride is obtained;-   (iv) subsequently setting the conditions in the crystallization zone    at a second temperature and at a second pressure so that    over-saturation with respect to gallium-containing nitride is    obtained and crystallization of gallium-containing nitride occurs on    the at least one crystallization seed and simultaneously setting the    conditions in the dissolution zone at a first temperature and at a    first pressure so that the second component of the    gallium-containing feedstock is dissolved;    wherein the second temperature is higher than the first temperature.

A gallium-containing nitride crystal obtainable by one of theseprocesses is also described. Further subject matter of the invention area gallium-containing nitride crystal having a surface area of more than2 cm² and having a dislocation density of less than 10^(6/cm) ² and agallium-containing nitride crystal having a thickness of at least 200 μmand a full width at half maximum (FWHM) of X-ray rocking curve from(0002) plane of 50 arcsec or less.

The invention also provides an apparatus for obtaining agallium-containing nitride crystal comprising an autoclave 1 having aninternal space and comprising at least one device 4, 5 for heating theautoclave to at least two zones having different temperatures, whereinthe autoclave comprises a device which separates the internal space intoa dissolution zone 13 and a crystallization zone 14.

In a yet another embodiment, a process for preparing a bulkmonocrystalline gallium-containing nitride in an autoclave is disclosed,which comprises the steps of providing a supercritical ammonia solutioncontaining gallium-containing nitride with ions of alkali metals, andrecrystallizing said gallium-containing nitride selectively on acrystallization seed from said supercritical ammonia solution by meansof the negative temperature coefficient of solubility and/or by means ofthe positive pressure coefficient of solubility.

A process for controlling recrystallization of a gallium-containingnitride in a supercritical ammonia solution which comprises steps ofproviding a supercritical ammonia solution containing agallium-containing nitride as a gallium complex with ions of alkalimetal and NH₃ solvent in an autoclave and decreasing the solubility ofsaid gallium-containing nitride in the supercritical ammonia solution ata temperature less than that of dissolving gallium-containing nitridecrystal and/or at a pressure higher than that of dissolvinggallium-containing nitride crystal is also disclosed.

FIG. 1 shows the dependency of the solubility of gallium-containingnitride in supercritical ammonia that contains potassium amide (withKNH₂:NH₃=0.07) on pressure at T=400° C. and T=500° C.

FIG. 2 shows the diagram of time variations of temperature in anautoclave at constant pressure for Example 1.

FIG. 3 shows the diagram of time variations of pressure in an autoclaveat constant temperature for Example 2.

FIG. 4 shows the diagram of time variations of temperature in anautoclave at constant volume for Example 3.

FIG. 5 shows the diagram of time variations of temperature in anautoclave for Example 4.

FIG. 6 shows the diagram of time variations of temperature in anautoclave for Example 5.

FIG. 7 shows the diagram of time variations of temperature in anautoclave for Example 6.

FIG. 8 shows the diagram of time variations of temperature in anautoclave for Example 7.

FIG. 9 shows a schematic axial cross section of an autoclave as employedin many of the examples, mounted in the furnace.

FIG. 10 is a schematic perspective drawing of an apparatus according tothe present invention.

FIG. 11 shows the diagram of time variations of temperature in anautoclave at constant volume for Example 8.

FIG. 12 shows the diagram of time variations of temperature in anautoclave at constant volume for Example 9.

FIG. 13 shows the diagram of time variations of temperature in anautoclave at constant volume for Example 10.

FIG. 14 shows the diagram of time variations of temperature in anautoclave at constant volume for Examples 11 and 12.

FIG. 15 illustrates the postulated theory of the invention.

FIG. 16 shows the diagram of time variations of temperature in anautoclave at constant volume for Example 13.

In the present invention the following definitions apply:

Gallium-containing nitride means a nitride of gallium and optionallyother element(s) of group XII (according to IUPAC, 1989). It includes,but is not restricted to, the binary compound GaN, ternary compoundssuch as AlGaN, InGaN and also AlInGaN (The mentioned formulas are onlyintended to give the components of the nitrides. It is not intended toindicate their relative amounts).

Bulk monocrystalline gallium-containing nitride means a monocrystallinesubstrate made of gallium-containing nitride from which e.g.optoelectronic devices such as LED or LD can be formed by epitaxialmethods such as MOCVD and HVPE.

Supercritical solvent means a fluid in a supercritical state. It canalso contain other components in addition to the solvent itself as longas these components do not substantially influence or disturb thefunction of the supercritical solvent. In particular, the solvent cancontain ions of alkali metals.

Supercritical solution is used when referring to the supercriticalsolvent when it contains gallium in a soluble form originating from thedissolution of gallium-containing feedstock.

Dissolution of gallium-containing feedstock means a process (eitherreversible or irreversible) in which said feedstock is taken up into thesupercritical solvent as gallium in a soluble form, possibly as galliumcomplex compounds.

Gallium complex compounds are complex compounds, in which a gallium atomis a coordination center surrounded by ligands, such as NH₃ molecules orits derivatives, like NH₂ ⁻, NH²⁻, etc.

Negative temperature coefficient of solubility means that the solubilityof a respective compound is a monotonically decreasing function oftemperature if all other parameters are kept constant. Similarly,positive pressure coefficient of solubility means that, if all otherparameters are kept constant, the solubility is a monotonicallyincreasing function of pressure. In our research we showed that thesolubility of gallium-containing nitride in supercriticalnitrogen-containing solvents, such as ammonia possesses a negativetemperature coefficient and a positive pressure coefficient intemperatures ranging at least from 300 to 600° C. and pressures from 1to 5.5 kbar.

Over-saturation of supercritical solution with respect togallium-containing nitride means that the concentration of gallium in asoluble form in said solution is higher than that in equilibrium (i.e.it is higher than solubility). In the case of dissolution ofgallium-containing nitride in a closed system, such an over-saturationcan be achieved by either increasing the temperature and/or decreasingthe pressure.

Spontaneous crystallization means an undesired process where nucleationand growth of the gallium-containing nitride from over-saturatedsupercritical solution take place at any site within an autoclave exceptat the surface of a crystallization seed where the growth is desired.Spontaneous crystallization also comprises nucleation and disorientedgrowth on the surface of crystallization seed.

Selective crystallization on a seed means a process of crystallizationon a seed carried out without spontaneous crystallization.

Autoclave means a closed container which has a reaction chamber wherethe ammonobasic process according to the present invention is carriedout.

The present invention can provide a gallium-containing nitridemonocrystal having a large size and a high quality. Suchgallium-containing nitride crystals can have a surface area of more than2 cm² and a dislocation density of less than 10⁶/cm². Gallium-containingnitride crystals having a thickness of at least 200 μm (preferably atleast 500 μm) and a FWHM of 50 arcsec or less can also be obtained.Depending on the crystallization conditions, it possible to obtaingallium-containing nitride crystals having a volume of more than 0.05cm³, preferably more than 0.1 cm³ using the processes of the invention.

As was explained above, the gallium-containing nitride crystal is acrystal of nitride of gallium and optionally other element(s) of GroupXIII (the numbering of the groups is given according to the IUPACconvention of 1989 throughout this application). These compounds can berepresented by the formula Al_(x)Ga_(1-x-y)In_(y)N, wherein 0≦x<1,0≦y<1, 0≦x+y<1 (preferably 0≦x<0.5 and 0≦y<0.5). Although in a preferredembodiment, the gallium-containing nitride is gallium nitride, in afarther preferred embodiment part (e.g. up to 50 mol.-%) of the galliumatoms can be replaced by one or more other elements of Group XIII(especially Al and/or In).

The gallium-containing nitride may additionally include at least onedonor and/or at least one acceptor and/or at least one magnetic dopante.g. to alter the optical, electrical and magnetic properties of thesubstrate. Donor dopants, acceptor dopants and magnetic dopants arewell-known in the art and can be selected according to the desiredproperties of the substrate. Preferably the donor dopants are selectedfrom the group consisting of Si and O. As acceptor donors Mg and Zn arepreferred. Any known magnetic dopant can be included into the substratesof the present invention. A preferred magnetic dopant is Mn and possiblyalso Ni and Cr. The concentrations of the dopants are well-known in theart and depend on the desired end application of the nitride. Typicallythe concentrations of these dopants range from 10¹⁷ to 10²¹/cm³. Insteadof adding dopants as part of the feedstock into the autoclave, dopantscan also be included into the gallium-containing nitride crystal fromtrace amounts of the autoclave material which dissolve during theprocess of the invention. For example, if the autoclave comprises anickel alloy then nickel can be included into the gallium-containingnitride crystal.

Due to the preparation process the gallium-containing nitride crystalcan also contain alkali elements, usually in an amount of more thanabout 0.1 ppm. Generally it is desired to keep the alkali elementscontent lower than 10 ppm, although it is difficult to specify whatconcentration of alkali metals in gallium-containing nitride has adisadvantageous influence on its properties.

It is also possible that halogens are present in the gallium-containingnitride. The halogens can be introduced either intentionally (as acomponent of the mineralizer) or unintentionally (from impurities of themineralizer or the feedstock). It is usually desired to keep the halogencontent of the gallium-containing nitride crystal in the range of about0.1 ppm or less.

The process of the invention is a supercritical crystallization process,which includes at least two steps: a dissolution step at a firsttemperature and at a first pressure and a crystallization step at asecond temperature and at a second pressure. Since generally highpressures and/or high temperatures are involved, the process accordingto the invention is preferably conducted in an autoclave. The two steps(i.e. the dissolution step and the crystallization step) can either beconducted separately or can be conducted at least partiallysimultaneously in the same reactor.

For conducting the two steps separately, the process can be conducted inone single reactor but the dissolution step is conducted before thecrystallization step. In this embodiment the reactor can have theconventional construction of a single chamber. The process of theinvention in the two-step embodiment can be conducted using constantpressure and two different temperatures or using constant temperatureand two different pressures. It is also possible to use two differentpressures and two different temperatures. The exact values of pressureand temperature should be selected depending on the feedstock, thespecific nitride to be prepared and the solvent. Generally the pressureis in the range of 1 to 10 kbar, preferably 1 to 5.5 kbar and morepreferably 1.5 to 3 kbar. The temperature is usually in the range of100° C. to 800° C., preferably 300° C. to 600° C., more preferably 400°C. to 550° C. If two different pressures are employed, the difference inpressure should be from 0.1 kbar to 9 kbar, preferably from 0.2 kbar to3 kbar. However, if the dissolution and crystallization are controlledby the temperature, the difference in temperature should be at least 1°C., and preferably from 5° C. to 150° C.

In a preferred embodiment, the dissolution step and the crystallizationstep are conducted at least partially simultaneously in the samecontainer. For such an embodiment the pressure is practically uniformwithin the container, while the temperature difference between thedissolution zone and crystallization zone should be at least 1° C., andpreferably is from 5° C. to 150° C. Furthermore, the temperaturedifference between the dissolution zone and crystallization zone shouldbe controlled so as to ensure chemical transport in the supercriticalsolution, which takes place through convection.

A possible construction of a preferred container is given in FIG. 9. Forconciseness and ease of understanding in the following, the process willbe explained particularly with respect to this preferred embodiment.However, the invention can be conducted with different containerconstructions as long as the principles outlined in the specificationand the claims are adhered to.

In a preferred embodiment of the invention, the process can be conductedin an apparatus comprising an autoclave 1 having an internal space andcomprising at least one device 4, 5 for heating the autoclave to atleast two zones having different temperatures, wherein the autoclavecomprises a device which separates the internal space into a dissolutionzone 13 and a crystallization zone 14 (hereinafter also referred to as“separating device” or “installation”). These two zones having differenttemperatures should preferably coincide with the dissolution zone 13 andthe crystallization zone 14. The device which separates the internalspace of the autoclave can be, for example, at least one baffle 12having at least one opening 2. Examples are baffles having a centralopening, circumferential openings or a combination thereof. The size ofthe opening(s) 2 should be large enough to allow transport between thezones but should be sufficiently small to maintain a temperaturegradient in the reactor. The appropriate size of the opening(s) dependson the size and the construction of the reactor and can be easilydetermined by a person skilled in the art.

In one embodiment, two different heating devices can be employed, theposition of which preferably corresponds to the dissolution zone 13 andthe crystallization zone 14. However, it has been observed thattransport of gallium in a soluble form from the dissolution zone 13 tothe crystallization zone 14 can be further improved if a cooling means 6is present between the first and the second heating devices and islocated at approximately the position of the separating device. Thecooling means 6 can be realized by liquid (e.g. water) cooling orpreferably by fan cooling. The heating devices can be poweredelectrically, by either inductive or, preferably, by resistive heatingmeans. Use of a heating-cooling-heating configuration gives widerpossibilities in forming the desired temperature distribution within theautoclave. For example, it enables to obtain a low temperature gradientswith in most of the crystallization zone 14 and a low temperaturegradient within most of the dissolution zone 13, while achieving a hightemperature gradient in the region of the baffle 12.

When the process of the present, invention is conducted agallium-containing feedstock, an alkali metal-containing component, atleast one crystallization seed and a nitrogen-containing solvent areprovided in at least one container. In the preferred apparatus describedabove, the gallium-containing feedstock 16 is placed in the dissolutionzone 13 and the at least one crystallization seed 17 is placed in thecrystallization zone 14. The alkali metal-containing component is alsopreferably placed in the dissolution zone. Then the nitrogen-containingsolvent is added to the container, which is then closed. Subsequentlythe nitrogen-containing solvent is brought into a supercritical state,e.g. by increasing pressure and/or heat.

In the present invention any materials containing gallium, which aresoluble in the supercritical solvent under the conditions of the presentinvention, can be used as a gallium-containing feedstock. Typically thegallium-containing feedstock will be a substance or mixture ofsubstances, which contains at least gallium, and optionally alkalimetals, other Group XIII elements, nitrogen, and/or hydrogen, such asmetallic Ga, alloys and inter-metallic compounds, hydrides, amides,imides, amido-imides, azides. Suitable gallium-containing feedstocks canbe selected from the group consisting of gallium nitride GaN, azidessuch as Ga(N₃)₃, imides such as Ga₂(NH)₃, amido-imides such asGa(NH)NH₂, amides such as Ga(NH₂)₃, hydrides such as GaH₃,gallium-containing alloys, metallic gallium and mixtures thereof.Preferred feedstocks are metallic gallium and gallium nitride andmixtures thereof. Most preferably, the feedstock is metallic gallium andgallium nitride. If elements of Group XIII other than gallium are to bepresent in the gallium-containing nitride crystal, correspondingcompounds or mixed compounds including Ga and the other Group XIIIelement can be used. If the substrate is to contain dopants or otheradditives, precursors thereof can be added to the feedstock.

The form of the feedstock is not particularly limited and it can be inthe form of one or more pieces or in the form of a powder. If thefeedstock is in the form of a powder, care should be taken thatindividual powder particles are not transported from the dissolutionzone to the crystallization zone, where they can cause irregularcrystallization. It is preferable that the feedstock is in one or morepieces and that the surface area of the feedstock is larger than that ofthe crystallization seed.

The nitrogen-containing solvent employed in the present invention mustbe able to form a supercritical fluid, in which gallium can be dissolvedin the presence of alkali metal ions. Preferably the solvent is ammonia,a derivative thereof or mixtures thereof. An example of a suitableammonia derivative is hydrazine. Most preferably the solvent is ammonia.To reduce corrosion of the reactor and to avoid side-reactions, halogense.g. in the form of halides are preferably not intentionally added intothe container. Although traces of halogens may be introduced into thesystem in the form of unavoidable impurities of the starting materials,care should be taken to keep the amount of halogen as low as possible.Due to the use of a nitrogen-containing solvent such as ammonia it isnot necessary to include nitride compounds into the feedstock. Metallicgallium (or aluminium or indium) can be employed as the feedstock whilethe solvent provides the nitrogen required for the nitride formation.

It has been observed that the solubility of gallium-containingfeedstock, such as gallium and corresponding elements of Group XIIIand/or their compounds, can be significantly improved by the presence ofat least one type of alkali metal-containing component as asolubilization aid (“mineralizer”). Lithium, sodium and potassium arepreferred as alkali metals, wherein sodium and potassium are morepreferred. The mineralizer can be added to the supercritical solvent inelemental form or preferably in the form of its compound (such as asalt). Generally the choice of the mineralizer depends on the solventemployed in the process. According to our investigations, alkali metalhaving a smaller ion radius can provide lower solubility ofgallium-containing nitride in the supercritical solvent than thatobtained with alkali metals having a larger ion radius. For example, ifthe mineralizer is in the form of a compound such as a salt, it ispreferably in the form of an alkali metal hydride such as MH, an alkalimetal nitride such as M₃N, an alkali metal amide such as MNH₂, an alkalimetal imide such as M₂NH or an alkali metal azide such as MN₃ (wherein Mis an alkali metal). The concentration of the mineralizer is notparticularly restricted and is selected so as to ensure adequate levelsof solubility of both feedstock (the starting material) andgallium-containing nitride (the resulting product). It is usually in therange of 1:200 to 1:2, in the terms of the mols of the metal ion basedon the mols of the solvent (molar ratio). In a preferred embodiment theconcentration is from 1:100 to 1:5, more preferably 1:20 to 1:8 mols ofthe metal ion based on the mols of the solvent.

The presence of the alkali metal ions in the process can lead to alkalimetal in the thus prepared substrates. It is possible that the amount ofalkali metal is more than about 0.1 ppm, even more than 10 ppm. However,in these amounts the alkali metals do not detrimentally effect theproperties of the substrates. It has been found that even at an alkalimetal content of 500 ppm, the operational parameters of the substrateaccording to the invention are still satisfactory.

The dissolved feedstock crystallizes in the crystallization step underthe low solubility conditions on the crystallization seed(s) which areprovided in the container. The process of the invention allows bulkgrowth of monocrystalline gallium-containing nitride on thecrystallization seed(s) and in particular leads to the formation ofstoichiometric gallium-containing nitride in the form of a bulkmonocrystalline layer on the crystallization seed(s).

Various crystals can be used as crystallization seeds in the presentinvention. however, it is preferred that the chemical andcrystallographic constitution of the crystallization seeds is similar tothose of the desired layer of bulk monocrystalline gallium-containingnitride. Therefore, the crystallization seed preferably comprises acrystalline layer of gallium-containing nitride. To facilitatecrystallization of the dissolved feedstock, the dislocation density ofthe crystallization seed is preferably less than 10⁶/cm². Suitablecrystallization seeds generally have a surface area of 8×8 mm² or moreand thickness of 100 μm or more, and can be obtained e.g. by HVPE.

After the starting materials have been introduced into the container andthe nitrogen-containing solvent has been brought into its supercriticalstate, the gallium-containing feedstock is at least partially dissolvedat a first temperature and a first pressure, e.g. in the dissolutionzone of an autoclave. Gallium-containing nitride crystallizes on thecrystallization seed (e.g. in the crystallization zone of an autoclave)at a second temperature and at a second pressure while thenitrogen-containing solvent is in the supercritical state, wherein thesecond temperature is higher than the first temperature and/or thesecond pressure is lower than the first pressure. If the dissolution andthe crystallization steps take place simultaneously in the samecontainer, the second pressure is essentially equal to the firstpressure.

This is possible since the solubility of gallium-containing nitrideunder the conditions of the present invention shows a negativetemperature coefficient and a positive pressure coefficient in thepresence of alkali metal ions. Without wishing to be bound by theory, itis postulated that the following processes occur. In the dissolutionzone, the temperature and pressure are selected such that thegallium-containing feedstock is dissolved and the nitrogen-containingsolution is undersaturated with respect to gallium-containing nitride.In the crystallization zone, the temperature and pressure are selectedsuch that the solution, although it contains approximately the sameconcentration of gallium as in the dissolution zone, is over-saturatedwith respect to gallium-containing nitride. Therefore, crystallizationof gallium-containing nitride on the crystallization seed occurs. Thisis illustrated in FIG. 15. Due e.g. to the temperature gradient,pressure gradient, concentration gradient, different chemical orphysical character of dissolved feedstock and crystallized product etc.,gallium is transported in a soluble form from the dissolution zone tothe crystallization zone. In the present invention this is referred toas “chemical transport” of gallium-containing nitride in thesupercritical solution. It is postulated that the soluble form ofgallium is a gallium complex compound with a Ga atom in the coordinationcenter surrounded by ligands, such as NH₃ molecules or its derivatives,like NH₂ ⁻, NH²⁻, etc.

This theory is equally applicable for all gallium-containing nitrides,such as AlGaN. InGaN and AlInGaN as well as GaN (The mentioned formulasare only intended to give the components of the nitrides. It is notintended to indicate their relative amounts). In the case of nitridesother than gallium nitride aluminum and/or indium in a soluble form alsohave to be present in the supercritical solution.

In a preferred embodiment of the invention, the gallium-containingfeedstock is dissolved in at least two steps. In this embodiment, thegallium-containing feedstock generally comprises two kinds of startingmaterials which differ in solubility. The difference in solubility canbe achieved chemically (e.g. by selecting two different chemicalcompounds) or physically (e.g. by selecting two forms of the samecompound having for example different surface areas, likemicrocrystalline powder and large crystals). In a preferred embodiment,the gallium-containing feedstock comprises two different chemicalcompounds such as metallic gallium and gallium nitride which dissolve atdifferent rates. In a first dissolution step, the first component of thegallium-containing feedstock is substantially completely dissolved at adissolution temperature and at a dissolution pressure in the dissolutionzone. The dissolution temperature and the dissolution pressure, whichcan be set only in the dissolution zone or preferably in the wholecontainer, are selected so that the second component of thegallium-containing feedstock and the crystallization seed(s) remainsubstantially undissolved. This first dissolution step results in anundersaturated or at most saturated solution (preferably undersaturatedsolution) with respect to gallium-containing nitride. For example, thedissolution temperature can be 100° C. to 350° C., preferably from 150°C. to 300° C. The dissolution pressure can be 0.1 kbar to 5 kbar,preferably from 0.1 kbar to 3 kbar.

Subsequently the conditions in the crystallization zone are set at asecond temperature and at a second pressure so that over-saturation withrespect to gallium-containing nitride is obtained and crystallization ofgallium-containing nitride occurs on the at least one crystallizationseed. Simultaneously the conditions in the dissolution zone are set at afirst temperature and at a first pressure (preferably equal to thesecond pressure) so that the second component of the gallium-containingfeedstock is now dissolved (second dissolution step). As explained abovethe second temperature is higher than the first temperature and/or thesecond pressure is lower than the first pressure so that thecrystallization can take advantage of the negative temperaturecoefficient of solubility and/or of the positive pressure coefficient ofsolubility. Preferably the first temperature is higher than thedissolution temperature. During the second dissolution step and thecrystallization step, the system should be in a stationary state so thatthe concentration of gallium in the supercritical solution remainssubstantially constant, i.e. approximately the same amount of galliumshould be dissolved per unit of time as is crystallized in the same unitof time. This allows for the growth of gallium-containing nitridecrystals of especially high quality and large size.

Typical pressures for the crystallization step and the seconddissolution step are in the range of 1 to 10 kbar, preferably 1 to 5.5kbar and more preferably 1.5 to 3 kbar. The temperature is generally inthe range of 100 to 800° C., preferably 300 to 600° C., more preferably400 to 550° C. The difference in temperature should be at least 1° C.,and is preferably from 5° C. to 150° C. As explained above, thetemperature difference between the dissolution zone and crystallizationzone should be controlled so as to ensure chemical transport in thesupercritical solution, which takes place through convection.

In the process of the invention, the crystallization should take placeselectively on the crystallization seed and not on a wall of thecontainer. Therefore, the over-saturation extent with respect to thegallium-containing nitride in the supercritical solution in thecrystallization zone should be controlled so as to be below thespontaneous crystallization level where crystallization takes place on awall of the autoclave and/or disoriented growth occurs on the seed, i.e.the level at which spontaneous crystallization occurs. This can beachieved by adjusting the chemical transport rate and/or thecrystallization temperature and/or crystallization pressure. Thechemical transport is related on the speed of a convective flow from thedissolution zone to the crystallization zone, which can be controlled bythe temperature difference between the dissolution zone and thecrystallization zone, the size of the opening(s) of baffle(s) betweenthe dissolution zone and the crystallization zone etc.

The performed tests showed that the best bulk monocrystalline galliumnitride obtained had a dislocation density close to 10⁴/cm² andsimultaneously a FWHM of X-ray rocking curve from (0002) plane below 60arcsec. These crystals possess an appropriate quality and durability foroptical semiconductor devices. The gallium-containing nitride of thepresent invention typically has a wurzite structure.

Feedstock material for use in the present invention can also be preparedusing a method similar to those described above. The method involves thesteps of:

-   -   (i) providing a gallium-containing feedstock, an alkali        metal-containing component, at least one crystallization seed        and a nitrogen-containing solvent in a container having at least        one zone;    -   (ii) subsequently bringing the nitrogen-containing solvent into        a supercritical state;    -   (iii) subsequently dissolving the gallium-containing feedstock        (such as metallic gallium or aluminium or indium, preferably        metallic gallium) at a dissolution temperature and at a        dissolution pressure, whereby the gallium-containing feedstock        is substantially completely dissolved and the crystallization        seed remains substantially undissolved so that an undersaturated        solution with respect to gallium-containing nitride is obtained;    -   (iv) subsequently setting the conditions in at least part of the        container at a second temperature and at a second pressure so        that over-saturation with respect to gallium-containing nitride        is obtained and crystallization of gallium-containing nitride        occurs on the at least one crystallization seed;    -   wherein the second temperature is higher than the dissolution        temperature.

In this embodiment the comments given above with respect to theindividual components, process parameters, etc. also apply. Preferablyduring the crystallization step in this embodiment the conditions in thewhole container are set at the second temperature and the secondpressure.

Gallium-containing nitride exhibits good solubility in a supercriticalnitrogen-containing solvent (e.g. ammonia), provided alkali metals ortheir compounds, such as KNH₂, are introduced into it. FIG. 1 shows thesolubility of gallium-containing nitride in a supercritical solventversus pressure for temperatures of 400 and 500° C. wherein thesolubility is defined by the molar percentage:S_(m)≡GaN^(solvent):(KNH₂+NH₃) 100%. In the present case the solvent issupercritical ammonia containing KNH₂ in a molar ratio x≡KNH₂:NH₃ equalto 0.07. For this case S_(m) should be a smooth function of only threeparameters: temperature, pressure, and molar ratio of mineralizer (i.e.S_(m)=S_(m)(T, p, x)). Small changes of S_(m) can be expressed as:ΔS _(m)≈(∂S _(m) /∂T)|_(p,x) ΔT+(∂S _(m) /∂p)|_(T,x) Δp+(∂S _(m)/∂x)|_(T,p) Δx,where the partial differentials (e.g. (∂S_(m)/∂T)|_(p,x)) determine thebehavior of S_(m) with variation of its parameters (e.g. T). In thisspecification the partial differentials are called “coefficients” (e.g.(∂S_(m)/∂T)|_(p,x) is a “temperature coefficient of solubility” or“temperature coefficient”).

The diagram shown in FIG. 1 illustrates that the solubility increaseswith pressure and decreases with temperature, which means that itpossesses a negative temperature coefficient and a positive pressurecoefficient. Such features allow obtaining a bulk monocrystallinegallium-containing nitride by dissolution in the higher solubilityconditions, and crystallization in the lower solubility conditions. Inparticular, the negative temperature coefficient means that, in thepresence of a temperature gradient, the chemical transport of gallium ina soluble form can take place from the dissolution zone having a lowertemperature to the crystallization zone having a higher temperature.

The process according to invention allows the growth of bulkmonocrystalline gallium-containing nitride crystals on thecrystallization seed and leads in particular to the formation ofstoichiometric gallium-containing nitride, obtained in the form of abulk monocrystalline layer grown on a gallium-containing nitridecrystallization seed. Since such a monocrystal is obtained in asupercritical solution that contains ions of alkali metals, it cancontain alkali metals in a quantity higher than 0.1 ppm. Because it isdesired to maintain a purely basic character of the supercriticalsolution, mainly in order to avoid corrosion of the apparatus, halidesare preferably not intentionally introduced into the solvent. Theprocess of the invention can also provide a bulk monocrystallinegallium-containing nitride crystal in which part of the gallium, e.g.from 5 to 50 mol-% may be substituted by Al and/or In. Moreover, thebulk monocrystalline gallium-containing nitride crystal may be dopedwith donor and/or acceptor and/or magnetic dopants. These dopants canmodify optical, electric and magnetic properties of thegallium-containing nitride crystal. With respect to the other physicalproperties, the bulk monocrystalline gallium-containing nitride crystalcan have a dislocation density below 10⁶/cm², preferably below 10⁵/cm²;or most preferably below 10⁴/cm². Besides, the FWHM of the X-ray rockingcurve from (0002) plane can be below 600 arcsec, preferably below 300arcsec, and most preferably below 60 arcsec. The best bulkmonocrystalline gallium nitride obtained may have a dislocation densitylower than 10⁴/cm² and simultaneously a FWHM of the X-ray rocking curvefrom (0002) plane below 60 arcsec.

Due to their good crystalline quality the gallium-containing nitridecrystals obtained in the present invention may be used as a substratematerial for optoelectronic semiconductor devices based on nitrides, inparticular for laser diodes.

The following examples are intended to illustrate the invention andshould not be construed as being limiting.

EXAMPLES

The dislocation density can be measured by th eso-called EPD method(Etch Pit Density) and subsequent evaluation using a microscope

The FWHM of the X-ray rocking curve can be determined by X-raydiffraction analysis.

Since it is not possible to readily measure the temperature in anautoclave while in use under supercritical conditions, the temperaturein the autoclave was estimated by the following method. The outside ofthe autoclave is equipped with thermocouples near the dissolution zoneand the crystallization zone. For the calibration, additionalthermocouples were introduced into the inside of the empty autoclave inthe dissolution zone and the crystallization zone. The empty autoclavewas then heated stepwise to various temperatures and the values of thetemperature of the thermocouples inside the autoclave and outside theautoclave were measured and tabulated. For example, if the temperatureof the crystallization zone is determined to be 500° C. and thetemperature of the dissolution zone is 400° C. inside the emptyautoclave when the temperature measured by the outside thermocouples are480° C. and 395° C., respectively. It is assumed that undersupercritical conditions the temperatures in thecrystallization/dissolution zones will also be 500° C./400° C. whentemperatures of 480° C./395° C. are measured by the outsidethermocouples. In reality, the temperature difference between the twozones can be lower due to effective heat transfer through thesupercritical solution.

Example 1

Two crucibles were placed into a high-pressure autoclave having a volumeof 10.9 cm³. The autoclave was manufactured according to a known design[H. Jacobs, D. Schmidt, Current Topics in Materials Science, vol. 8, ed.E. Kaldis (North-Holland, Amsterdam, 1981); 381]. One of the cruciblescontained 0.4 g of gallium nitride in the form of 0.1 mm thick platesproduced by the HVPE method as feedstock, while the other contained agallium nitride seed of a double thickness weighing 0.1 g. The seed wasalso obtained by the HVPE method. Further, 0.72 g of metallic potassiumof 4N purity was placed in the autoclave, the autoclave was filled with4.81 g of ammonia and then closed. The autoclave was put into a furnaceand heated to a temperature of 400° C. The pressure within the autoclavewas 2 kbar. After 8 days the temperature was increased to 500° C., whilethe pressure was maintained at the 2 kbar level and the autoclave wasmaintained under these conditions for another 8 days.(FIG. 2). As aresult of this process, in which the dissolution and crystallizationsteps were separated in time, the feedstock was completely dissolved andthe recrystallization of gallium nitride in the form of a layer tookplace on the partially dissolved seed. The two-sided monocrystallinelayers had a total thickness of about 0.4 mm.

Example 2

Two crucibles were put into the above-mentioned high-pressure autoclavehaving a volume of 10.9 cm³. One of the crucibles contained 0.44 g ofgallium nitride in the form of 0.1 mm thick plates produced by the HVPEmethod as feedstock, and the other contained a gallium nitride seed of adouble thickness weighing 0.1 g, also obtained by the HVPE method.Further, 0.82 g of metallic potassium of 4N purity was placed in theautoclave, the autoclave was filled with 5.43 g of ammonia and thenclosed. The autoclave was put into a furnace and heated to a temperatureof 500° C. The pressure within the autoclave was 3.5 kbar. After 2 daysthe pressure was lowered to 2 kbar, while the temperature was maintainedat the 500° C. level and the autoclave was maintained under theseconditions for another 4 days (FIG. 3). As a result of this process, thefeedstock was completely dissolved and the recrystallization of galliumnitride took place on the partially dissolved seed. The two-sidedmonocrystalline layers had a total thickness of about 0.25 mm.

Example 3

Two crucibles were placed into the above-mentioned high-pressureautoclave having a volume of 10.9 cm³. One of the crucibles contained0.3 g of the feedstock in the form of metallic gallium of 6N purity andthe other contained a 0.1 g gallium nitride seed obtained by the HVPEmethod. Further, 0.6 g of metallic potassium of 4N purity was placed inthe autoclave; the autoclave was filled with 4 g of ammonia and thenclosed. The autoclave was put into a furnace and heated to a temperatureof 200° C. After 2 days the temperature was increased to 500° C., whilethe pressure was maintained at the 2 kbar level and the autoclave wasmaintained in these conditions for further 4 days (FIG. 4). As a resultof this process, the feedstock was completely dissolved and thecrystallization of gallium nitride took place on the seed. The two-sidedmonocrystalline layers had a total thickness of about 0.3 mm.

Example 4

This is an example of a process, in which the dissolution andcrystallization steps take place simultaneously (recrystallizationprocess). In this example and all the following an apparatus is usedwhich is schematically shown in FIG. 9 and FIG. 10. The basic unit ofthe apparatus is the autoclave 1, which in this Example has a volume of35.6 cm³. The autoclave 1 is equipped with an separating device 2 whichallows for chemical transport of the solvent in the supercriticalsolution inside the autoclave 1. For this purpose, the autoclave 1 isput into a chamber 3 of a set of two furnaces 4 provided with heatingdevices 5 and a cooling device 6. The autoclave 1 is secured in adesired position with respect to the furnaces 4 by means of a screw-typeblocking device 7. The furnaces 4 are mounted on a bed 8 and are securedby means of steel tapes 9 wrapped around the furnaces 4 and the bed 8.The bed 8 together with the set of furnaces 4 is rotationally mounted inbase 10 and is secured in a desired angular position by means of a pininterlock 11. In the autoclave 1, placed in the set of furnaces 4, theconvective flow of supercritical solution takes place as determined bythe separating device 2. The separating device 2 is in the form of ahorizontal baffle 12 having a circumferential opening. The baffle 12separates the dissolution zone 13 from the crystallization zone 14 inthe autoclave 1, and enables, together with the adjustable tilting angleof the autoclave 1, controlling of speed and type of convective flow.The temperature level of the individual zones in the autoclave 1 iscontrolled by means of a control system 15 operating the furnaces 4. Inthe autoclave 1, the dissolution zone 13 coincides with thelow-temperature zone of the set of furnaces 4 and is located above thehorizontal baffle 12 and the feedstock 16 is put into this zone 13. Onthe other hand, the crystallization zone 14 coincides with thehigh-temperature zone of the set of furnaces 4 and it is located belowthe horizontal baffle 12. The crystallization seed 17 is mounted in thiszone 14. The mounting location of the crystallization seed 17 is belowthe intersection of the rising and descending convective streams.

An amount of 3.0 g of gallium nitride produced by the HVPE method wasplaced in the high-pressure autoclave described above, which was set inthe horizontal position. This gallium nitride had the form of plates ofabout 0.2 mm thickness, and it was distributed (roughly uniformly) inequal portions in the dissolution zone 13 and the crystallization zone14. The portion placed in the dissolution zone 13 played the role offeedstock, whereas the portion placed in the crystallization zone 14played the role of crystallization seeds. Metallic potassium of 4Npurity was also added in a quantity of 2.4 g. Then the autoclave 1 wasfilled with 15.9 g of ammonia (5N); closed, put into a set of furnaces 4and heated to a temperature of 450° C. The pressure inside the autoclave1 was approx. 2 kbar. During this stage, which lasted one day, partialdissolution of gallium nitride was carried out in both zones. Then thetemperature of the crystallization zone 14 was increased to 500° C.while the temperature of the dissolution zone 13 was lowered to 400° C.and the autoclave 1 was kept in these conditions for 6 more days (FIG.5). As a final result of this process, partial dissolution of thefeedstock in the dissolution zone 13 and crystallization of galliumnitride on the gallium nitride seeds in the crystallization zone 14 tookplace.

Example 5

The above-mentioned high pressure autoclave 1 having a volume of 35.6cm³ was charged with feedstock in the form of a 3.0 g pellet of sinteredgallium nitride (introduced into the dissolution zone 13), two seeds ofgallium nitride obtained by the HVPE method and having the form ofplates having a thickness of 0.4 mm and a total weight of 0.1 g(introduced into the crystallization zone 14), as well as with 2.4 g ofmetallic potassium of 4N purity. Then the autoclave was filled with 15.9g of ammonia (5N) and closed. The autoclave 1 was then put into a set offurnaces 4 and heated to 450° C. The pressure inside the autoclave wasabout 2 kbar. After an entire day the temperature of the crystallizationzone 14 was raised to 480° C., while the temperature of the dissolutionzone 13 was lowered to 420° C. and the autoclave was maintained underthese conditions for 6 more days (see FIG. 6). As a result of theprocess the feedstock was partially dissolved in the dissolution zone 13and gallium nitride crystallized on the seeds in the crystallizationzone 14. The two-sided monocrystalline layers had a total thickness ofabout 0.2 mm.

Example 6

The above-mentioned high pressure autoclave 1 having a volume of 35.6cm³ (see FIG. 9) was charged with 1.6 g of feedstock in the form ofgallium nitride produced by the HVPE method and having the form ofplates having a thickness of about 0.2 mm (introduced into thedissolution zone 13), three gallium-nitride seeds of a thickness ofabout 0.35 mm and a total weight of 0.8 g, also obtained by the HVPEmethod (introduced into the crystallization zone 14), as well as with3.56 g of metallic potassium of 4N purity. The autoclave 1 was filledwith 14.5 g of ammonia (5N) and closed. Then the autoclave 1 was putinto a set of furnaces 4 and heated to 425° C. The pressure inside theautoclave was approx. 1.5 kbar. After an entire day the temperature ofthe dissolution zone 13 was lowered to 400° C. while the temperature ofthe crystallization zone 14 was increased to 450° C. and the autoclavewas kept in these conditions for 8 more days (see FIG. 7). After theprocess, the feedstock was found to be partially dissolved in thedissolution zone 13 and gallium nitride had crystallized on the seeds ofthe HVPE GaN in the crystallization zone 14. The two-sidedmonocrystalline layers had a total thickness of about 0.15 mm.

Example 7

The above-mentioned high pressure autoclave 1 having a volume of 35.6cm³ (see FIG. 9) was charged in its dissolution zone 13 with 2 g offeedstock in the form of gallium nitride produced by the HVPE method andhaving the form of plates having a thickness of about 0.2 mm. and 0.47 gof metallic potassium of 4N purity, and in its crystallization zone 14with three GaN seeds of a thickness of about 0.3 mm and a total weightof about 0.3 g also obtained by the HVPE method. The autoclave wasfilled with 16.5 g of ammonia (5N) and closed. Then the autoclave 1 wasput into a set of furnaces 4 and heated to 500° C. The pressure insidethe autoclave was approx. 3 kbar. After an entire day the temperature inthe dissolution zone 13 was reduced to 450° C. while the temperature inthe crystallization zone 14 was raised to 550° C. and the autoclave waskept under these conditions for the next 8 days (see FIG. 8). After theprocess, the feedstock was found to be partially dissolved in thedissolution zone 13 and gallium nitride had crystallized on the seeds inthe crystallization zone 14. The two-sided monocrystalline layers had atotal thickness of about 0.4 mm.

Example 8

An amount of 1.0 g of gallium nitride produced by the HVPE method wasput into the dissolution zone 13 of the high-pressure autoclave 1 havinga volume of 35.6 cm³. In the crystallization zone 14 of the autoclave, acrystallization seed of gallium nitride having a thickness of 100 μm anda surface area of 2.5 cm², obtained by the HVPE method, was placed. Thenthe autoclave was charged with 1.2 g of metallic gallium of 6N purityand 2.2 g of metallic potassium of 4N purity. Subsequently, theautoclave 1 was filled with 15.9 g of ammonia (5N), closed, put into aset of furnaces 4 and heated to a temperature of 200° C. After 3days—during which period metallic gallium was dissolved in thesupercritical solution—the temperature was increased to 450° C. whichresulted in a pressure of about 2.3 kbar. The next day thecrystallization zone temperature was increased to 500° C. while thetemperature of the dissolution zone 13 was lowered to 370° C. and theautoclave 1 was kept in these conditions for the next 20 days (see FIG.11). As a result of this process, the partial dissolution of thematerial in the dissolution zone 13 and the growth of the galliumnitride on the gallium nitride seed in the crystallization zone 14 tookplace. The resulting crystal of gallium nitride having a total thicknessof 350 μm was obtained in the form of two-sided monocrystalline layers.

Example 9

An amount of 3.0 g of gallium nitride in the form of a sintered galliumnitride pellet was put into the dissolution zone 13 of high-pressureautoclave 1 having a volume of 35.6 cm³ (see FIG. 9). In thecrystallization zone 14 of the autoclave, a crystallization seed ofgallium nitride obtained by the HVPE method and having a thickness of120 μm and a surface area of 2.2 cm² was placed. Then the autoclave wascharged with 2.3 g of metallic potassium of 4N purity. Subsequently, theautoclave 1 was filled with 15.9 g of ammonia (5N), closed, put into aset of furnaces 4 and heated to a temperature of 250° C. in order topartially dissolve the sintered GaN pellet and obtain a preliminarysaturation of a supercritical solution with gallium in a soluble form.After two days, the temperature of the crystallization zone 14 wasincreased to 500° C. while the temperature of the dissolution zone 13was lowered to 420° C. and the autoclave 1 was kept in these conditionsfor the next 20 days (see FIG. 12). As a result of this process, partialdissolution of the material in the dissolution zone 13 and growth ofgallium nitride on the gallium nitride seed took place in thecrystallization zone 14. A crystal of gallium nitride having a totalthickness of 500 μm was obtained in the form of two-sidedmonocrystalline layers.

Example 10

An amount of 0.5 g of gallium nitride plates having an average thicknessof about 120 μm, produced by the HVPE method, were put into thedissolution zone 13 of high-pressure autoclave 1 having a volume of 35.6cm³. In the crystallization zone 14 of the autoclave, threecrystallization seeds of gallium nitride obtained by the HVPE methodwere placed. The crystallization seeds had a thickness of about 120 μmand a total surface area of 1.5 cm². Then the autoclave was charged with0.41 g of metallic lithium of 3N purity. Subsequently, the autoclave 1was filled with 14.4 g of ammonia (5N), closed, put into a set offurnaces 4 and heated so that the temperature of the crystallizationzone 14 was increased to 550° C. and the temperature of the dissolutionzone 13 was increased to 450° C. The resulting pressure was about 2.6kbar. The autoclave 1 was kept in these conditions for the next 8 days(see FIG. 13). As a result of this process, partial dissolution of thematerial in the dissolution zone 13 and growth of gallium nitride on thegallium nitride seeds in the crystallization zone 14 took place. Theresulting crystals of gallium nitride had a thickness of 40 μm and werein the form of two-sided monocrystalline layers.

Example 11

An amount of 0.5 g of gallium nitride having an average thickness ofabout 120 μm, produced by the HVPE method, was placed into thedissolution zone 13 of hill-pressure autoclave 1 having a volume of 35.6cm³. In the crystallization zone 14 of the autoclave, threecrystallization seeds of gallium nitride obtained by the HVPE methodwere placed. The crystallization seeds had a thickness of 120 μm and atotal surface area of 1.5 cm². Then the autoclave was charged with 0.071g of metallic gallium of 6N purity and 1.4 g of metallic sodium of 3Npurity. Subsequently, the autoclave 1 was filled with 14.5 g of ammonia(5N), closed, put into a set of furnaces 4 and heated to a temperatureof 200° C. After 1 day—during which period metallic gallium wasdissolved in the supercritical solution—the autoclave 1 was heated sothat the temperature in the crystallization zone was increased to 500°C., while the temperature in the dissolution zone was increased to 400°C. The resulting pressure was about 2.3 kbar. The autoclave 1 was keptin these conditions for the next 8 days (see FIG. 14). As a result ofthis process, partial dissolution of the material in the dissolutionzone 13 and growth of gallium nitride on the gallium nitride seeds inthe crystallization zone 14 took place. The resulting crystals ofgallium nitride were obtained in the form of two-sided monocrystallinelayers having a total thickness of 400 μm.

Example 12

An amount of 0.5 g of gallium nitride having an average thickness ofabout 120 μm, produced by the HVPE method, was placed into thedissolution zone 13 of the high-pressure autoclave 1 having a volume of35.6 cm³. In the crystallization zone 14 of the autoclave, threecrystallization seeds of gallium nitride obtained by the HVPE methodwere placed. The crystallization seeds had a thickness of 120 μm and atotal surface area of 1.5 cm². Then the autoclave was charged with 0.20g of gallium amide and 1.4 g of metallic sodium of 3N purity.Subsequently, the autoclave 1 was filled with 14.6 g of ammonia (5N),closed, put into a set of furnaces 4 and heated to a temperature of 200°C. After 1 day—during which period gallium amide was dissolved in thesupercritical solution—the autoclave 1 was heated so that thetemperature in the crystallization zone was increased to 500° C., whilethe temperature in the dissolution zone was increased to 400° C. Theresulting pressure was about 2.3 kbar. The autoclave 1 was kept in theseconditions for the next 8 days (see also FIG. 14). As a result of thisprocess, partial dissolution of the material in the dissolution zone 13and growth of gallium nitride on the gallium nitride seeds in thecrystallization zone 14 took place. The resulting crystals of galliumnitride were in the form of two-sided monocrystalline layers having atotal thickness of 490 μm.

Example 13

One crucible was placed into the above-mentioned high-pressure autoclavehaving a volume of 10.9 cm³. The crucible contained 0.3 g of thefeedstock in the form of metallic gallium of 6N purity. Also threegallium-nitride seeds having a thickness of about 0.5 mm and a totalmass of 0.2 g, all obtained by the HVPE method, were suspended withinthe autoclave. Further, 0.5 g of metallic sodium of 3N purity was placedin the autoclave; the autoclave was filled with 5.9 g of ammonia andthen closed. The autoclave was put into a furnace and heated to atemperature of 200° C., where the pressure was about 2.5 kbar. After 1day the temperature was increased to 500° C., while the pressureincreased up to 5 kbar and the autoclave was maintained in theseconditions for further 2 days (FIG. 16). As a result of this process,the feedstock was completely dissolved and crystallization of galliumnitride took place on the seed. The average thickness of thetwo-side-overgrown monocrystalline layer of gallium nitride was about0.14 mm. The FWHM of the X-ray rocking curve from the (0002) plane atthe gallium-terminated side was 43 arcsec, while at thenitrogen-terminated side it was 927 arcsec.

The monocrystalline gallium nitride layers have a wurzite structure likein all of the other examples.

1. A process for obtaining a bulk monocrystalline gallium-containingnitride crystal, wherein it is performed in an autoclave, in theenvironment of a supercritical solvent containing ions of alkali metals,wherein a gallium-containing feedstock for making saidgallium-containing nitride crystal becomes dissolved in saidsupercritical solvent to form a supercritical solution, and thegallium-containing nitride becomes crystallized from the supercriticalsolution on the surface of a crystallization seed at a temperaturehigher and/or pressure lower than that of the feedstock dissolution inthe supercritical solvent.
 2. The process according to claim 1, whereinsaid process comprises the steps of dissolving the gallium-containingfeedstock and a separate step of transferring the supercritical solutionto the higher temperature and/or to the lower pressure.
 3. The processaccording to claim 1, wherein said process comprises the step ofsimultaneous creation of at least two zones of different temperatures,said gallium-containing feedstock is placed in the dissolution zone ofthe lower temperature, while the crystallization seed is placed in thecrystallization zone of the higher temperature.
 4. The process accordingto claim 3, wherein said temperature difference between said dissolutionzone and said crystallization zone is controlled so as to ensurechemical transport in the supercritical solution.
 5. The processaccording to claim 4, wherein said chemical transport in thesupercritical solution takes place through convection in the autoclave.6. The process according to claim 4, wherein said temperature differencebetween the dissolution zone and the crystallization zone is greaterthan 1° C.
 7. The process according to claim 1, wherein saidgallium-containing nitride crystal has the formulaAl_(x)Ga_(1-x-y)In_(y)N, where 0≦x<1, 0≦y<1, 0x+y<1.
 8. The processaccording to claim 1, wherein said gallium-containing nitride crystalcontains dopants of a donor and/or acceptor and/or magnetic type.
 9. Theprocess according to claim 1, wherein said supercritical solventcontains NH₃ and/or its derivatives.
 10. The process according to claim1, wherein said supercritical solvent contains sodium and/or potassiumions.
 11. The process according to claim 1, wherein saidgallium-containing feedstock consists essentially of gallium-containingnitride and/or its precursors.
 12. The process according to claim 11,wherein said precursors are selected from the group consisting ofgallium azides, gallium imides, gallium amido-imides, gallium amides,gallium hydrides, gallium-containing alloys, and metallic gallium andoptionally corresponding compounds of other elements of Group XIII(according to IUPAC, 1989).
 13. The process according to claim 1,wherein said crystallization seed has at least a crystalline layer ofgallium-containing nitride.
 14. The process according to claim 1,wherein said crystallization seed has at least a crystalline layer ofgallium-containing nitride with a dislocation density below 10⁶/cm². 15.The process according to claim 1, wherein said crystallization of agallium-containing nitride takes place at the temperatures from 100 to800° C., preferably 300 to 600° C., more preferably 400 to 550° C. 16.The process according to claim 1, wherein said crystallization of agallium-containing nitride takes place at the pressures from 100 to10000 bar, preferably 1000 to 5500 bar, more preferably 1500 to 3000bar.
 17. The process according to claim 1, wherein the content of alkalimetal ions in the supercritical solvent is controlled so as to provideadequate levels of solubility of said feedstock as well as saidgallium-containing feedstock.
 18. The process according to claim 1,wherein the molar ratio of the moles of said alkali metal ions to themoles of the supercritical solvent is controlled within the range of1:200 to 1:2, preferably 1:100 to 1:5, more preferably 1:20 to 1:8. 19.An apparatus for obtaining of a monocrystalline gallium-containingnitride crystal, comprising an autoclave for producing supercriticalsolvent, equipped with installation for establishing a convection flow,the autoclave being mounted inside a furnace or set of furnaces whichare equipped with heating devices and/or cooling devices, wherein saidfurnace or set of furnaces has a high-temperature zone coinciding withthe crystallization zone of said autoclave equipped with heating devicesand/or cooling devices, as well as a low-temperature zone coincidingwith the dissolution zone of the autoclave equipped with heating devicesand/or cooling devices, wherein feedstock is placed in the autoclave inthe dissolution zone, and said a crystallization seed is placed in thecrystallization zone, and said convection flow between the zones isestablished by said installation, and wherein said dissolution zone islocated above said horizontal baffle or horizontal baffles, whereas saidcrystallization zone is located below said horizontal baffle orhorizontal baffles.
 20. The apparatus according to claim 19, whereinsaid installation is in the form of a horizontal baffle or horizontalbaffles having central and/or circumferential openings, separating thecrystallization zone from the dissolution zone.